The present invention relates generally to processes for low temperature thermal decomposition of waste plastics. Specifically, the invention focuses upon achieving decomposition of waste plastics at a lower temperature than was previously possible. In particular municipal, health and industrial waste plastics are processed such as (but not limited to) polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), polyurethane (PU), and polyvinyl chloride (PVC).
Waste plastics, that is synthetic polymer-containing substances, pose an environmental issue because of the problems associated with disposal: a large volume of non-biodegradable material. Because of the limits on landfill capacity, future recycling or decomposition is a necessity. Direct recycling back to the manufacture is not always feasible because such waste plastic is often mixed with respect to polymer type and separation is uneconomical. Economical considerations for processing waste plastic often require the use of the unseparated mixed waste plastic. Plastic recycling originated with the manufacture of synthetic thermoplastics. Rejected parts, trim, and flash from operations represented valuable materials that were ground and recycled with virgin material. This process was potentially repeated a number of times provided the percentage of regrinds remained low. As long as the plastic scrap generated by the industry was clean and uncontaminated with other plastics, reprocessing within the industry continued to expand, provided the price of virgin plastic remained high. After 1960 with a decrease in prices, profit margins for plastic scrap were squeezed, and disposal instead of reprocessing often occurred.
After 1970, plastic prices rose again due to OPEC raising the cost of petroleum feedstocks and recycling practices again increased. Interest increased not only in processes for reclaiming waste plastics, such as product evaluation for chemicals and fuels, but also in the necessary step of separation of plastics from other waste material. A review of this early history of plastics recycling is given by R. J. Ehrig in Plastics Recycling, Oxford University Press, NY, 1992, hereinafter referred to as Ehrig (1992). Some of the early operating plants for recycled plastic included a Department of Energy funded plant in LaPorte, Tex., which used a fluidized bed of sand and was designed for 17 million pounds per year of atactic polypropylene. It ran from 1980-82. In 1984 at Ebenhausen, Germany, a 20 million pound per year plant used molten salt with a fluidized bed reactor to process plastic wastes and tires.
In all cases economics governed whether such plants continued operation. Since 1985 plastics recycling has become more economically feasible due to continued plastics technological growth and increased environmental concern, however, significant cost impacts remain due to the level of the elevated temperatures previously required.